Test strips are often used for qualitative and quantitative analysis of blood components or other fluids. With the lateral flow method, a spatial separation is defined in the strips between the sample application area and detection zone. Most conventional lateral flow strips are designed for test samples that are readily available in large quantities (e.g., urine). However, when the test sample is blood, the collection of a large sample may cause undue pain to the patient. Thus, one technique that has been utilized to accommodate smaller test sample volumes is to “spot” the sample directly on the membrane surface of the test strip. Thereafter, a diluent is used to wash away the test sample and carry it to the detection zone. Unfortunately, variations associated with sample transfer and diffusion of the sample to the membrane result in a flow that is largely uncontrolled and uneven before reaching the detection zone. This may have an adverse effect on the accuracy of the device because the amount of analyte and/or label captured across the detection zone is not consistent at the time of measurement.
In addition, various tests on blood samples require separation of the red blood cell components from the sample to obtain plasma or serum that is essentially free of red blood cells. The sample can then be used in various assays without interference from red blood cell components. In this regard, filter arrangements have been proposed for production of serum or plasma from whole blood. For example, U.S. Pat. No. 5,423,989 describes a membrane filtering arrangement with a first coarse membrane coated with a fibrous protein and a second fine membrane for removing red blood cells from a test sample.
As such, a need currently exists for a simple and efficient technique for metering and filtering a low volume blood test sample such that a known volume of blood plasma or serum may be easily transferred to a detection zone of a lateral flow assay device.